The peripheral nervous system comprises pairs of cord-like nerves arising from the brain and the spinal cord and includes both a somatic portion and an autonomic portion. In contrast to the somatic portion, the autonomic portion of the peripheral nervous system is not subject to direct voluntary control. It is the autonomic nervous system that controls functions such as digestion, heart rate, and respiratory rate. To provide this control, the autonomic nervous system (ANS) is in turn composed of the sympathetic and parasympathetic nervous systems. These two systems are thought to act somewhat in opposition to each other. The sympathetic nervous system controls “fight or flight” reflexes whereas the parasympathetic system controls “rest and digest” functions. While this is most certainly an over-simplification, there is no doubt that these two components of the ANS have different functions and exert control over a wide range of organ systems that operate below the level of conscious control. Furthermore, imbalances of the autonomic nervous system have been associated with a wide range of disease states. The best characterized of these is cardiac autonomic dysfunction, which is characterized by increased activity of sympathetic nervous system activity and decreased activity of the parasympathetic nervous system.
A large body of evidence has conclusively demonstrated that patients with congestive heart failure (CHF) have decreased activity of the parasympathetic nervous system and that these changes have been observed following acute myocardial infarction, and in patients with ischemic heart disease. Indeed, loss of parasympathetic nervous system activity is an independent predictor of sudden death, development of lethal cardiac arrhythmias and the likelihood of adverse cardiac events. In addition to its role in cardiac disease, imbalances of the ANS have been associated with a number of other disorders including the metabolic syndrome, Type 1 and 2 diabetes, autoimmune disorders, and anxiety disorders (such as post-traumatic stress disorder or PTSD).
To address imbalance within the ANS, neuromodulation of the vagus nerve has been used. The vagus nerve, like most other cranial nerves, arises in pairs from the brain stem. The vagus nerve includes some somatic fibers for controlling speech but is largely a parasympathetic nerve that enervates assorted organs and muscles including the heart. The parasympathetic control of the heart through the vagus nerve slows the heart rate whereas sympathetic control increases the heart rate. Given this relationship between the vagus nerves and heart rate, various approaches have been developed to electrically stimulate one or both of the vagus nerves to decrease heart rate. But the vagus nerve arises from the brain stem and passes through the carotid sheath in the neck before enervating the viscera in the chest cavity and abdomen. It is thus not readily stimulated using transcutaneous electrodes but instead requires electrode implantation. An implantation on the vagus nerve is inherently quite invasive given the relatively inaccessible location of the vagus nerve. Thus, electrode implantations for vagus nerve stimulation have assorted health risks and dangers. But the need for neurostimulation to balance ANS activity is well known with regard to cardiac health.
Accordingly, there is a need in the art for less invasive neurostimulation techniques for balancing ANS activity.